1. Field of the Invention
This invention relates generally to a process and apparatus for the remediation of contaminated soils, sediments and sludges, and other granular and particulate materials.
More specifically, this invention relates to a process for removing those contaminants which display a significant vapor pressure at elevated temperatures from soils and like materials and to apparatus for carrying out that process. In one particular embodiment, this invention is directed to the cleaning of soil by the removal of volatile organic compounds such as chlorinated solvents, petroleum hydrocarbons and pesticides therefrom.
2. Description of the Related Art
The contamination of soils, sediments and like materials with industrial solvents, petroleum hydrocarbons and other organic liquids is of increasing concern. Contamination of soil by such compounds occurs in a variety of ways including spills and discharges during processing, loading, storage and transport, percolation into the soil from waste lagoons and landfills, pipeline breaks, leakage from underground storage tanks, agricultural uses and from dumping. One harm from such contamination is in the degradation of ground water and ground water aquifers especially those used as a source of potable water. In extreme instances, pollution of ground water has been so extensive as to preclude use of an aquifer as a municipal water supply. A second harm occurs when compounds migrate-or volatilize and threaten the health and wellbeing of nearby residents.
Regulations governing the treatment of contaminated soils generally require excavation and treatment of those soils contaminated with more than 1,000 ppm total petroleum hydrocarbons (TPH) by weight. Soils treated to levels below 100 ppm can be disposed of in a non-toxic landfill and, if treated to a level below 10 ppm, can be returned to the ground from which the soils were taken. Certain hazardous and toxic compounds, benzene, toluene and xylenes for example, are subject to far more stringent requirements and must be removed from soils to a level below 5 ppb before the soil is returned to the ground.
There are numerous approaches to the treatment of soils to remove contaminants. One common approach is to pile the soil on a prepared pad arranged to allow for air or other gas to be drawn downwardly through the pile. The air as it passes through the soil strips volatile organic compounds from the soil and carries those compounds with it. Air is collected from pipes located in the soil pile and is then treated to remove the volatile organic compounds. The concentration of stripped organic compounds carried in the exiting air stream is quite low; often on the order of a few parts per million. A very large volume of air must then be treated during the course of a soil remediation project. Typical approaches to the treatment of such air streams includes passing the air through a column containing an adsorbent solid such as activated carbon or by fume incineration of the contaminant compounds. This latter approach, although applied to an air stream resulting from the stripping of volatile organic compounds from ground water rather than from the soil itself, is illustrated by U.S. Pat. No. 4,892,664. This is a slow and lengthy process requiring a large amount of space and energy.
Incineration of contaminated soils has been used. It is technically feasible to destroy most contaminant compounds by incineration but the technique is not practical for large soil volumes and is very costly requiring large quantities of fuel. All of the soil must be raised to incineration temperatures leaving a product which is hot and difficult to handle and to transport.
U.S. Pat. No. 4,974,528 describes what might be called a modified incineration process. Patentee uses a vehicle mounted, inclined rotary kiln for removing hydrocarbon contaminants from soils. Contaminated soil is fed into the upper end of the kiln and a burner assembly is located at the lower, or discharge, end of the kiln. Soil is heated to temperatures as high as 345.degree. C. as it passes through the kiln resulting in the volatilization or burning of the lighter fraction hydrocarbons from the soil. Combustion gases are removed from the upper kiln end and are drawn through a bag house for the removal of fines. The cleaned gases are then reheated and passed to a catalytic incinerator to burn the remaining hydrocarbons in the gas.
A number of other approaches to soil remediation have been compiled by the Environmental Protection Agency in a publication entitled "The Superfund Innovative Technology Evaluation Program: Technology Profiles"; EPA/540/5-89/013, November 1989. Among the processes listed therein, the following are considered worthy of note. A vapor extraction system under development by American Toxic Disposal, Inc of Waukegan, Ill. employs a fluidized bed to remove volatile contaminants from soils and similar materials. Direct contact between the soil and a gas from a gas fired heater at a temperature of about 320 F. volatilizes water and contaminants from the soil into the gas stream. Particulates are first removed from the gas stream by means of a cyclone separator and baghouse. The gas stream from the baghouse is cooled in a venturi scrubber, countercurrent washer and chiller and is then passed through a carbon bed to adsorb the remaining contaminants. Most of the soil fed to the fluidized bed is recovered as a clean, dry dust cleaned of volatiles. Other by-products include a small quantity of sludge resulting from clarification of the water used in the process; quantities of spent adsorbent carbon; a wastewater stream that might require further treatment; and small amounts of baghouse and cyclone dust. It is stated that the process can remove polychlorinated biphenyls, polynuclear aromatic hydrocarbons, Volatile inorganics and some pesticides from soils.
Another process described in the Environmental Protection Agency publication is a soil washing system which has been demonstrated by Biotrol, Inc. of Chaska, Minn. Contaminated soil is subjected to attrition washing during which highly contaminated fine soil particles are separated from the coarser sand and gravel. The bulk of the soil is discharged as a clean washed product leaving a process water which contains the highly contaminated fine particles as well as dissolved contaminants. Thereafter, the fine solids are dewatered to obtain a thickened slurry and a clarified process water stream. Both the slurry and the clarified water require a secondary treatment before discharge which may be, for example, a biodegradation process. The technique was developed to clean soils contaminated with wood preserving chemicals.
Yet another approach to the cleaning of contaminated soils was reported in a paper presented at the 84th Annual Meeting and Exhibition of the Air and Waste Management Association at Vancouver, British Columbia in June, 1991. The paper, entitled "Batch Steam Distillation/ Metal Extraction Treatment Process for Contaminated Superfund Soils", by E. S. Alperin et al of the IT Corporation, Knoxville, Tenn., which process was earlier reported in the Environmental Protection Agency publication referred to above. The paper describes a two-stage process, which in a first stage removes volatile organic compounds from a slurry of the contaminated soil by injecting low pressure steam directly into the slurry contained within a closed vessel. Steam containing contaminant compounds is taken overhead from the vessel and is condensed to recover the volatile organic compounds. After the steam distillation is complete, concentrated hydrochloric acid is added to the soil slurry to solubilize the metals. The acid solution is then separated from the soil which is thereafter washed several times with water, dewatered, and dried.
All of the soil remediation processes of the prior art have serious drawbacks. The use of air or some other gas to strip contaminant compounds from soils takes a long time, produces an uneven result as uniform percolation of air through the soil cannot easily be achieved, and requires the secondary treatment of very large air volumes. Incineration is very costly, produces a product that is difficult to handle and, in some instances, requires contaminant removal from the incinerator flue gas. The fluidized bed system also is energy intensive, requires extensive treatment of the exiting gas stream and leaves the soil in a hot dry, dusty state while producing several by-product streams which require further treatment. All incineration systems may also contribute to other regulated gaseous contaminants being released from fuel burning, such as SO.sub.x and NO.sub.x and CO.sub.2 which may have effects on global warming. Soil washing using attrition washing is necessarily limited to soils which are predominately sand and gravel and results as well in a silt or clay suspension which requires further treatment and is difficult to dewater. Batch low pressure steam distillation of slurries is high in energy use, limited in treatable compounds, and requires the handling of water-soil slurries with all the attendant difficulties entailed therewith.
There is clearly a need for additional simple, effective ways to clean up soils contaminated with solvents, hydrocarbons, and the like to reduce residual contaminants to low levels with efficient fuel usage without producing as well large volume byproduct liquid and/or gas streams requiring purification. Further, it is of considerable advantage to obtain as a product a soil suitable for replacement in the ground without further conditioning or treatment, which none of the above mentioned alternatives accomplish.